In Electronic Toll Collection (ETC) systems, Automatic Vehicle Identification (AVI) is achieved by the use of Radio Frequency (“RF”) communications between roadside readers and transponders within vehicles. Each reader continuously emits a coded identification signal and when a transponder enters into communication range and detects the reader the units transact information, in particular the unique identity of the transponder. In the USA, current AVI RF communication systems are licensed under the category of Location and Monitoring Systems (LMS) through the provisions of the Code of Federal Regulations (CFR) Title 47 Part 90 Subpart M.
The reader is typically connected to another controller, herein referred to as a Roadside Controller, which is also connected to a vehicle detector and an imaging system which work in association with the AVI RF system to permit all vehicles passing through the toll coverage to be detected, classified, and identified in order to permit the operator of the ETC system to apply appropriate charges to the owner of the vehicle. Those vehicles not equipped with transponders are typically photographed and the license plate numbers are analyzed to identify the vehicle. In ETC systems, it is generally necessary to determine in which lateral position a vehicle is traveling when it reaches the point of toll. For example, it is often necessary to separate vehicles equipped with transponders from vehicles without transponders and associate video images with vehicles that are not equipped. In other systems, the lanes may be equipped with physical barriers that will only be opened on valid transponder identification for the specific lanes. In order to do so in any of these systems, the ETC system must clearly identify where the subject vehicle is located within the multiple zones of coverage of the system.
Current ETC systems can be classed as either lane-based or open-road.
In a lane-based system, the reader controls reader channels, each of which corresponds to RF coverage of an individual vehicle lane, which will then communicate with vehicles in individual lanes. The RF communication coverage area of each channel is often referred to as the capture zone. In a lane-based system the capture zone is typically 1.2 to 2.4 meters (4-8 feet) long and 3 meters (10 feet) wide. Lane-based systems also require that the vehicles be laterally constrained to the lanes through appropriate physical measures such as barriers between lanes. Thus when a vehicle with a transponder passes through a capture zone, the vehicle location is easily associated with the specific lane at that instant in time, and the short length of the zone allows for accurate timing alignment with the vehicle detection imaging systems.
Open-road systems in contrast allow traffic to free flow without impediment of lane barriers. Thus vehicles may be laterally located anywhere over multiple lanes of traffic, for example they can be mid-way between two lanes, and moreover need not be traveling parallel to the lanes, for example they can be changing lanes as they pass through the toll area.
Current open-road toll ETC systems can be classed either as open-lane-based or locator-based.
Open-lane-based systems employ RF capture zones similar in size to the lane-based systems but the systems employ more channels than lanes to provide overlapping or staggered RF capture zones over multiple lanes. The reader analyses detections from multiple capture zones to determine to which zone to assign the vehicle location. An example open-lane-based ETC system in described in U.S. Pat. No. 6,219,613, which is owned in common herewith.
Locator-based systems in contrast use wide-area communications, where a single RF channel spans multiple traffic lanes in width and is also much longer than a lane-based system. The capture zone of locator-based systems is typically 16.8 meters (55 feet) wide by 36.6 meters (120 feet) long. One major difference is that, unlike the lane-based approaches, multiple transponders can be simultaneously present in the coverage area. The locator-based system typically uses two receivers, each with a separate antenna, to simultaneously receive signals from a transponder. By comparison of the properties of the signal received at the two receivers, such as amplitude difference, phase difference or time difference of arrival, and knowledge of the RF communication timing, the system can determine the vehicle location to a precision equal to that to the lane-based systems. The locator antenna system may operate in accord with the system described in U.S. Pat. No. 6,025,799, which is owned in common herewith.
One issue for ETC systems is synchronizing the RF communication system and the vehicle detection system. If the communication occurs too early or too late, then it is possible to wrongly associate another vehicle with the communicating transponder. Additionally, vehicle positions relative to the lanes can be changing as vehicles pass through the toll area, so that it is necessary that a communication occurs with the moving vehicle while the car is close to the vehicle detection point. At 70 mph (102 feet per second) a vehicle will typically only remain in the lane-based capture zone for less than 60 ms while in a locator-based system this time increases to around 1200 ms. It is noted that a toll transaction may require multiple information packets to be exchanged between the reader and the transponder, and this must occur during that short time.
To ensure this synchronization, current North American toll systems employ Time Division Multiple Access (TDMA) RF communications at nominally 900 MHz. Each information packet exchange—transmission of data and its acknowledgement—occurs over a period of a few milliseconds. The TDMA structure allows the reader to interrogate specific transponders at time instants controlled by the reader, thereby allowing the reader to synchronize the data exchange with the transponder with the timing of the other roadside equipment.
The potential exists to perform the RF communications with a non-TDMA, more general purpose wide area communication system. In particular, in the US, the CFR 47 provisions allow for vehicular and roadside communications under Parts 90 and 95 in the category Dedicated Short Range Communications (DSRC) Service at nominally 5.9 GHz using an extension of the IEEE 802.11 communication standard as specified currently under ASTM E2213. However, unlike LMS, DSRC communications permitted are not restricted to location and monitoring functions and can extend up to 400 m or more in range from the communication antenna.
The DSRC communication system is intended for sharing for multiple applications and, while 802.11 based communication systems support high data rates, there are inherent latencies in the communications and variable communications delays.
It would be advantageous to provide for an ETC system and method capable of vehicle detection employing a wide area communications protocol, in particular one with variable communication delays